Method of forming a multilayer film having a continuous and disperse phase

ABSTRACT

An optical film is provided which comprises a disperse phase of polymeric particles disposed within a continuous birefringent matrix. The film is oriented, typically by stretching, in one or more directions. The size and shape of the disperse phase particles, the volume fraction of the disperse phase, the film thickness, and the amount of orientation are chosen to attain a desired degree of diffuse reflection and total transmission of electromagnetic radiation of a desired wavelength in the resulting film.

This is a continuation of application Ser. No. 08/609,753 filed Feb. 29,1996 now U.S. Pat. No. 5,867,316.

FIELD OF THE INVENTION

This invention relates to optical materials which contain structuressuitable for controlling optical characteristics, such as reflectanceand transmission. In a further aspect, it relates to control of specificpolarizations of reflected or transmitted light.

BACKGROUND

Optical films are known to the art which are constructed from inclusionsdispersed within a continuous matrix. The characteristics of theseinclusions can be manipulated to provide a range of reflective andtransmissive properties to the film. These characteristics includeinclusion size with respect to wavelength within the film, inclusionshape and alignment, inclusion volumetric fill factor and the degree ofrefractive index mismatch with the continuous matrix along the film'sthree orthogonal axes.

Conventional absorbing (dichroic) polarizers have, as their inclusionphase, inorganic rod-like chains of light-absorbing iodine which arealigned within a polymer matrix. Such a film will tend to absorb lightpolarized with its electric field vector aligned parallel to therod-like iodine chains, and to transmit light polarized perpendicular tothe rods. Because the iodine chains have two or more dimensions that aresmall compared to the wavelength of visible light, and because thenumber of chains per cubic wavelength of light is large, the opticalproperties of such a film are predominately specular, with very littlediffuse transmission through the film or diffuse reflection from thefilm surfaces. Like most other commercially available polarizers, thesepolarizing films are based on polarization-selective absorption.

Films filled with inorganic inclusions with different characteristicscan provide other optical transmission and reflective properties. Forexample, coated mica flakes with two or more dimensions that are largecompared with visible wavelengths, have been incorporated into polymericfilms and into paints to impart a metallic glitter. These flakes can bemanipulated to lie in the plane of the film, thereby imparting a strongdirectional dependence to the reflective appearance. Such an effect canbe used to produce security screens that are highly reflective forcertain viewing angles, and transmissive for other viewing angles. Largeflakes having a coloration (specularly selective reflection) thatdepends on alignment with respect to incident light, can be incorporatedinto a film to provide evidence of tampering. In this application, it isnecessary that all the flakes in the film be similarly aligned withrespect to each other.

However, optical films made from polymers filled with inorganicinclusions suffer from a variety of infirmities. Typically, adhesionbetween the inorganic particles and the polymer matrix is poor.Consequently, the optical properties of the film decline when stress orstrain is applied across the matrix, both because the bond between thematrix and the inclusions is compromised, and because the rigidinorganic inclusions may be fractured. Furthermore, alignment ofinorganic inclusions requires process steps and considerations thatcomplicate manufacturing.

Other films, such as that disclosed in U.S. Pat. No. 4,688,900 (Doaneet. al.), consists of a clear light-transmitting continuous polymermatrix, with droplets of light modulating liquid crystals dispersedwithin. Stretching of the material reportedly results in a distortion ofthe liquid crystal droplet from a spherical to an ellipsoidal shape,with the long axis of the ellipsoid parallel to the direction ofstretch. U.S. Pat. No. 5,301,041 (Konuma et al.) make a similardisclosure, but achieve the distortion of the liquid crystal dropletthrough the application of pressure. A. Aphonin, "Optical Properties ofStretched Polymer Dispersed Liquid Crystal Films: Angle-DependentPolarized Light Scattering, Liquid Crystals, Vol. 19, No. 4, 469-480(1995), discusses the optical properties of stretched films consistingof liquid crystal droplets disposed within a polymer matrix. He reportsthat the elongation of the droplets into an ellipsoidal shape, withtheir long axes parallel to the stretch direction, imparts an orientedbirefringence (refractive index difference among the dimensional axes ofthe droplet) to the droplets, resulting in a relative refractive indexmismatch between the dispersed and continuous phases along certain filmaxes, and a relative index match along the other film axes. Such liquidcrystal droplets are not small as compared to visible wavelengths in thefilm, and thus the optical properties of such films have a substantialdiffuse component to their reflective and transmissive properties.Aphonin suggests the use of these materials as a polarizing diffuser forbacklit twisted nematic LCDs. However, optical films employing liquidcrystals as the disperse phase are substantially limited in the degreeof refractive index mismatch between the matrix phase and the dispersedphase. Furthermore, the birefringence of the liquid crystal component ofsuch films is typically sensitive to temperature.

U.S. Pat. No. 5,268,225 (Isayev) discloses a composite laminate madefrom thermotropic liquid crystal polymer blends. The blend consists oftwo liquid crystal polymers which are immiscible with each other. Theblends may be cast into a film consisting of a dispersed inclusion phaseand a continuous phase. When the film is stretched, the dispersed phaseforms a series of fibers whose axes are aligned in the direction ofstretch. While the film is described as having improved mechanicalproperties, no mention is made of the optical properties of the film.However, due to their liquid crystal nature, films of this type wouldsuffer from the infirmities of other liquid crystal materials discussedabove.

Still other films have been made to exhibit desirable optical propertiesthrough the application of electric or magnetic fields. For example,U.S. Pat. No. 5,008,807 (Waters et al.) describes a liquid crystaldevice which consists of a layer of fibers permeated with liquid crystalmaterial and disposed between two electrodes. A voltage across theelectrodes produces an electric field which changes the birefringentproperties of the liquid crystal material, resulting in various degreesof mismatch between the refractive indices of the fibers and the liquidcrystal. However, the requirement of an electric or magnetic field isinconvenient and undesirable in many applications, particularly thosewhere existing fields might produce interference.

Other optical films have been made by incorporating a dispersion ofinclusions of a first polymer into a second polymer, and then stretchingthe resulting composite in one or two directions. U.S. Pat. No.4,871,784 (Otonari et al. ) is exemplative of this technology. Thepolymers are selected such that there is low adhesion between thedispersed phase and the surrounding matrix polymer, so that anelliptical void is formed around each inclusion when the film isstretched. Such voids have dimensions of the order of visiblewavelengths. The refractive index mismatch between the void and thepolymer in these "microvoided" films is typically quite large (about0.5), causing substantial diffuse reflection. However, the opticalproperties of microvoided materials are difficult to control because ofvariations of the geometry of the interfaces, and it is not possible toproduce a film axis for which refractive indices are relatively matched,as would be useful for polarization-sensitive optical properties.Furthermore, the voids in such material can be easily collapsed throughexposure to heat and pressure.

Optical films have also been made wherein a dispersed phase isdeterministically arranged in an ordered pattern within a continuousmatrix. U.S. Pat. No. 5,217,794 (Schrenk) is exemplative of thistechnology. There, a lamellar polymeric film is disclosed which is madeof polymeric inclusions which are large compared with wavelength on twoaxes, disposed within a continuous matrix of another polymeric material.The refractive index of the dispersed phase differs significantly fromthat of the continuous phase along one or more of the laminate's axes,and is relatively well matched along another. Because of the ordering ofthe dispersed phase, films of this type exhibit strong iridescence(i.e., interference-based angle dependent coloring) for instances inwhich they are substantially reflective. As a result, such films haveseen limited use for optical applications where optical diffusion isdesirable.

There thus remains a need in the art for an optical material consistingof a continuous and a dispersed phase, wherein the refractive indexmismatch between the two phases along the material's three dimensionalaxes can be conveniently and permanently manipulated to achievedesirable degrees of diffuse and specular reflection and transmission,wherein the optical material is stable with respect to stress, strain,temperature differences, and electric and magnetic fields, and whereinthe optical material has an insignificant level of iridescence. Theseand other needs are met by the present invention, as hereinafterdisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating an optical body made inaccordance with the present invention, wherein the disperse phase isarranged as a series of elongated masses having an essentially circularcross-section;

FIG. 2 is a schematic drawing illustrating an optical body made inaccordance with the present invention, wherein the disperse phase isarranged as a series of elongated masses having an essentiallyelliptical cross-section;

FIGS. 3a-e are schematic drawings illustrating various shapes of thedisperse phase in an optical body made in accordance with the presentinvention;

FIG. 4a is a graph of the bidirectional scatter distribution as afunction of scattered angle for an oriented film in accordance with thepresent invention for light polarized perpendicular to orientationdirection;

FIG. 4b is a graph of the bidirectional scatter distribution as afunction of scattered angle for an oriented film in accordance with thepresent invention for light polarized parallel to orientation direction;and

FIG. 5 is a schematic representation of a multilayer film made inaccordance with the present invention.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a diffusely reflectivefilm or other optical body comprising a birefringent continuouspolymeric phase and a substantially nonbirefringent disperse phasedisposed within the continuous phase. The indices of refraction of thecontinuous and disperse phases are substantially mismatched (i.e.,differ from one another by more than about 0.05) along a first of threemutually orthogonal axes, and are substantially matched (i.e., differ byless than about 0.05) along a second of three mutually orthogonal axes.In some embodiments, the indices of refraction of the continuous anddisperse phases can be substantially matched or mismatched along, orparallel to, a third of three mutually orthogonal axes to produce amirror or a polarizer. Incident light polarized along, or parallel to, amismatched axis is scattered, resulting in significant diffusereflection. Incident light polarized along a matched axis is scatteredto a much lesser degree and is significantly spectrally transmitted.These properties can be used to make optical films for a variety ofuses, including low loss (significantly nonabsorbing) reflectivepolarizers for which polarizations of light that are not significantlytransmitted are diffusely reflected.

In a related aspect, the present invention relates to an optical film orother optical body comprising a birefringent continuous phase and adisperse phase, wherein the indices of refraction of the continuous anddisperse phases are substantially matched (i.e., wherein the indexdifference between the continuous and disperse phases is less than about0.05) along an axis perpendicular to a surface of the optical body.

In another aspect, the present invention relates to a composite opticalbody comprising a polymeric continuous birefringent first phase in whichthe disperse second phase may be birefringent, but in which the degreeof match and mismatch in at least two orthogonal directions is primarilydue to the birefringence of the first phase.

In still another aspect, the present invention relates to a method forobtaining a diffuse reflective polarizer, comprising the steps of:providing a first resin, whose degree of birefringence can be altered byapplication of a force field, as through dimensional orientation or anapplied electric field, such that the resulting resin material has, forat least two orthogonal directions, an index of refraction difference ofmore than about 0.05; providing a second resin, dispersed within thefirst resin; and applying said force field to the composite of bothresins such that the indices of the two resins are approximately matchedto within less than about 0.05 in one of two directions, and the indexdifference between first and second resins in the other of twodirections is greater than about 0.05. In a related embodiment, thesecond resin is dispersed in the first resin after imposition of theforce field and subsequent alteration of the birefringence of the firstresin.

In yet another aspect, the present invention relates to an optical bodyacting as a reflective polarizer with a high extinction ratio. In thisaspect, the index difference in the match direction is chosen as smallas possible and the difference in the mismatch direction is maximized.The volume fraction, thickness, and disperse phase particle size andshape can be chosen to maximize the extinction ratio, although therelative importance of optical transmission and reflection for thedifferent polarizations may vary for different applications.

In another aspect, the present invention relates to an optical bodycomprising a continuous phase, a disperse phase whose index ofrefraction differs from said continuous phase by greater than about 0.05along a first axis and by less than about 0.05 along a second axisorthogonal to said first axis, and a dichroic dye. The optical body ispreferably oriented along at least one axis. The dichroic dye improvesthe extinction coefficient of the optical body by absorbing, in additionto scattering, light polarized parallel to the axis of orientation.

In the various aspects of the present invention, the reflection andtransmission properties for at least two orthogonal polarizations ofincident light are determined by the selection or manipulation ofvarious parameters, including the optical indices of the continuous anddisperse phases, the size and shape of the disperse phase particles, thevolume fraction of the disperse phase, the thickness of the optical bodythrough which some fraction of the incident light is to pass, and thewavelength or wavelength band of electromagnetic radiation of interest.

The magnitude of the index match or mismatch along a particular axiswill directly affect the degree of scattering of light polarized alongthat axis. In general, scattering power varies as the square of theindex mismatch. Thus, the larger the index mismatch along a particularaxis, the stronger the scattering of light polarized along that axis.Conversely, when the mismatch along a particular axis is small, lightpolarized along that axis is scattered to a lesser extent and is therebytransmitted specularly through the volume of the body.

The size of the disperse phase also can have a significant effect onscattering. If the disperse phase particles are too small (i.e., lessthan about 1/30 the wavelength of light in the medium of interest) andif there are many particles per cubic wavelength, the optical bodybehaves as a medium with an effective index of refraction somewhatbetween the indices of the two phases along any given axis. In such acase, very little light is scattered. If the particles are too large,the light is specularly reflected from the particle surface, with verylittle diffusion into other directions. When the particles are too largein at least two orthogonal directions, undesirable iridescence effectscan also occur. Practical limits may also be reached when particlesbecome large in that the thickness of the optical body becomes greaterand desirable mechanical properties are compromised.

The shape of the particles of the disperse phase can also have an effecton the scattering of light. The depolarization factors of the particlesfor the electric field in the index of refraction match and mismatchdirections can reduce or enhance the amount of scattering in a givendirection. The effect can either add or detract from the amount ofscattering from the index mismatch, but generally has a small influenceon scattering in the preferred range of properties in the presentinvention.

The shape of the particles can also influence the degree of diffusion oflight scattered from the particles. This shape effect is generally smallbut increases as the aspect ratio of the geometrical cross-section ofthe particle in the plane perpendicular to the direction of incidence ofthe light increases and as the particles get relatively larger. Ingeneral, in the operation of this invention, the disperse phaseparticles should be sized less than several wavelengths of light in oneor two mutually orthogonal dimensions if diffuse, rather than specular,reflection is preferred.

Dimensional alignment is also found to have an effect on the scatteringbehavior of the disperse phase. In particular, it has been observed, inoptical bodies made in accordance with the present invention, thataligned scatterers will not scatter light symmetrically about thedirections of specular transmission or reflection as randomly alignedscatterers would. In particular, inclusions that have been elongated byorientation to resemble rods scatter light primarily along (or near) acone centered on the orientation direction and having an edge along thespecularly transmitted direction. For example, for light incident onsuch an elongated rod in a direction perpendicular to the orientationdirection, the scattered light appears as a band of light in the planeperpendicular to the orientation direction with an intensity thatdecreases with increasing angle away from the specular directions. Bytailoring the geometry of the inclusions, some control over thedistribution of scattered light can be achieved both in the transmissivehemisphere and in the reflective hemisphere.

The volume fraction of the disperse phase also affects the scattering oflight in the optical bodies of the present invention. Within certainlimits, increasing the volume fraction of the disperse phase tends toincrease the amount of scattering that a light ray experiences afterentering the body for both the match and mismatch directions ofpolarized light. This factor is important for controlling the reflectionand transmission properties for a given application. However, if thevolume fraction of the disperse phase becomes too large, lightscattering diminishes. Without wishing to be bound by theory, thisappears to be due to the fact that the disperse phase particles arecloser together, in terms of the wavelength of light, so that theparticles tend to act together as a smaller number of large effectiveparticles.

The thickness of the optical body is also an important control parameterwhich can be manipulated to affect reflection and transmissionproperties in the present invention. As the thickness of the opticalbody increases, diffuse reflection also increases, and transmission,both specular and diffuse, decreases.

While the present invention will often be described herein withreference to the visible region of the spectrum, various embodiments ofthe present invention can be used to operate at different wavelengths(and thus frequencies) of electromagnetic radiation through appropriatescaling of the components of the optical body. Thus, as the wavelengthincreases, the linear size of the components of the optical body areincreased so that the dimensions, measured in units of wavelength,remain approximately constant. Another major effect of changingwavelength is that, for most materials of interest, the index ofrefraction and the absorption coefficient change. However, theprinciples of index match and mismatch still apply at each wavelength ofinterest.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

As used herein, the terms "specular reflection" and "specularreflectance" refer to the reflectance of light rays into an emergentcone with a vertex angle of 16 degrees centered around the specularangle. The terms "diffuse reflection" or "diffuse reflectance" refer tothe reflection of rays that are outside the specular cone defined above.The terms "total reflectance" or "total reflection" refer to thecombined reflectance of all light from a surface. Thus, total reflectionis the sum of specular and diffuse reflection.

Similarly, the terms "specular transmission" and "speculartransmittance" are used herein in reference to the transmission of raysinto an emergent cone with a vertex angle of 16 degrees centered aroundthe specular direction. The terms "diffuse transmission" and "diffusetransmittance" are used herein in reference to the transmission of allrays that are outside the specular cone defined above. The terms "totaltransmission" or "total transmittance" refer to the combinedtransmission of all light through an optical body. Thus, totaltransmission is the sum of specular and diffuse transmission.

As used herein, the term "extinction ratio" is defined to mean the ratioof total light transmitted in one polarization to the light transmittedin an orthogonal polarization.

FIGS. 1-2 illustrate a first embodiment of the present invention. Inaccordance with the invention, a diffusely reflective optical film 10 orother optical body is produced which consists of a birefringent matrixor continuous phase 12 and a discontinuous or disperse phase 14. Thebirefringence of the continuous phase is typically at least about 0.05,preferably at least about 0.1, more preferably at least about 0.15, andmost preferably at least about 0.2.

The indices of refraction of the continuous and disperse phases aresubstantially matched (i.e., differ by less than about 0.05) along afirst of three mutually orthogonal axes, and are substantiallymismatched (i.e., differ by more than about 0.05) along a second ofthree mutually orthogonal axes. Preferably, the indices of refraction ofthe continuous and disperse phases differ by less than about 0.03 in thematch direction, more preferably, less than about 0.02, and mostpreferably, less than about 0.01. The indices of refraction of thecontinuous and disperse phases preferably differ in the mismatchdirection by at least about 0.07, more preferably, by at least about0.1, and most preferably, by at least about 0.2.

The mismatch in refractive indices along a particular axis has theeffect that incident light polarized along that axis will besubstantially scattered, resulting in a significant amount ofreflection. By contrast, incident light polarized along an axis in whichthe refractive indices are matched will be spectrally transmitted orreflected with a much lesser degree of scattering. This effect can beutilized to make a variety of optical devices, including reflectivepolarizers and mirrors.

The present invention provides a practical and simple optical body andmethod for making a reflective polarizer, and also provides a means ofobtaining a continuous range of optical properties according to theprinciples described herein. Also, very efficient low loss polarizerscan be obtained with high extinction ratios. Other advantages are a widerange of practical materials for the disperse phase and the continuousphase, and a high degree of control in providing optical bodies ofconsistent and predictable high quality performance.

Effect of Index Match/Mismatch

In the preferred embodiment, the materials of at least one of thecontinuous and disperse phases are of a type that undergoes a change inrefractive index upon orientation. Consequently, as the film is orientedin one or more directions, refractive index matches or mismatches areproduced along one or more axes. By careful manipulation of orientationparameters and other processing conditions, the positive or negativebirefringence of the matrix can be used to induce diffuse reflection ortransmission of one or both polarizations of light along a given axis.The relative ratio between transmission and diffuse reflection isdependent on the concentration of the disperse phase inclusions, thethickness of the film, the square of the difference in the index ofrefraction between the continuous and disperse phases, the size andgeometry of the disperse phase inclusions, and the wavelength orwavelength band of the incident radiation.

The magnitude of the index match or mismatch along a particular axisdirectly affects the degree of scattering of light polarized along thataxis. In general, scattering power varies as the square of the indexmismatch. Thus, the larger the index mismatch along a particular axis,the stronger the scattering of light polarized along that axis.Conversely, when the mismatch along a particular axis is small, lightpolarized along that axis is scattered to a lesser extent and is therebytransmitted specularly through the volume of the body.

FIGS. 4a-b demonstrate this effect in oriented films made in accordancewith the present invention. There, a typical Bidirectional ScatterDistribution Function (BSDF) measurement is shown for normally incidentlight at 632.8 nm. The BSDF is described in J. Stover, "OpticalScattering Measurement and Analysis" (1990). The BSDF is shown as afunction of scattered angle for polarizations of light bothperpendicular and parallel to the axis of orientation. A scattered angleof zero corresponds to unscattered (spectrally transmitted) light. Forlight polarized in the index match direction (that is, perpendicular tothe orientation direction) as in FIG. 4a, there is a significantspecularly transmitted peak with a sizable component of diffuselytransmitted light (scattering angle between 8 and 80 degrees), and asmall component of diffusely reflected light (scattering angle largerthan 100 degrees). For light polarized in the index mismatch direction(that is, parallel to the orientation direction) as in FIG. 4b, there isnegligible specularly transmitted light and a greatly reduced componentof diffusely transmitted light, and a sizable diffusely reflectedcomponent. It should be noted that the plane of scattering shown bythese graphs is the plane perpendicular to the orientation directionwhere most of the scattered light exists for these elongated inclusions.Scattered light contributions outside of this plane are greatly reduced.

If the index of refraction of the inclusions (i.e., the disperse phase)matches that of the continuous host media along some axis, then incidentlight polarized with electric fields parallel to this axis will passthrough unscattered regardless of the size, shape, and density ofinclusions. If the indices are not matched along some axis, then theinclusions will scatter light polarized along this axis. For scatterersof a given cross-sectional area with dimensions larger thanapproximately λ/30 (where λ is the wavelength of light in the media),the strength of the scattering is largely determined by the indexmismatch. The exact size, shape and alignment of a mismatched inclusionplay a role in determining how much light will be scattered into variousdirections from that inclusion. If the density and thickness of thescattering layer is sufficient, according to multiple scattering theory,incident light will be either reflected or absorbed, but nottransmitted, regardless of the details of the scatterer size and shape.

When the material is to be used as a polarizer, it is preferablyprocessed, as by stretching and allowing some dimensional relaxation inthe cross stretch in-plane direction, so that the index of refractiondifference between the continuous and disperse phases is large along afirst axis in a plane parallel to a surface of the material and smallalong the other two orthogonal axes. This results in a large opticalanisotropy for electromagnetic radiation of different polarizations.

Some of the polarizers within the scope of the present invention areelliptical polarizers. In general, elliptical polarizers will have adifference in index of refraction between the disperse phase and thecontinuous phase for both the stretch and cross-stretch directions. Theratio of forward to back scattering is dependent on the difference inrefractive index between the disperse and continuous phases, theconcentration of the disperse phase, the size and shape of the dispersephase, and the overall thickness of the film. In general, ellipticaldiffusers have a relatively small difference in index of refractionbetween the particles of the disperse and continuous phases. By using abirefringent polymer-based diffuser, highly elliptical polarizationsensitivity (i.e., diffuse reflectivity depending on the polarization oflight) can be achieved. At an extreme, where the index of refraction ofthe polymers match on one axis, the elliptical polarizer will be adiffuse reflecting polarizer.

Methods of Obtaining Index Match/Mismatch

The materials selected for use in a polarizer in accordance with thepresent invention, and the degree of orientation of these materials, arepreferably chosen so that the phases in the finished polarizer have atleast one axis for which the associated indices of refraction aresubstantially equal. The match of refractive indices associated withthat axis, which typically, but not necessarily, is an axis transverseto the direction of orientation, results in substantially no reflectionof light in that plane of polarization.

The disperse phase may also exhibit a decrease in the refractive indexassociated with the direction of orientation after stretching. If thebirefringence of the host is positive, a negative strain inducedbirefringence of the disperse phase has the advantage of increasing thedifference between indices of refraction of the adjoining phasesassociated with the orientation axis while the reflection of light withits plane of polarization perpendicular to the orientation direction isstill negligible. Differences between the indices of refraction ofadjoining phases in the direction orthogonal to the orientationdirection should be less than about 0.05 after orientation, andpreferably, less than about 0.02.

The disperse phase may also exhibit a positive strain inducedbirefringence. However, this can be altered by means of heat treatmentto match the refractive index of the axis perpendicular to theorientation direction of the continuous phase. The temperature of theheat treatment should not be so high as to relax the birefringence inthe continuous phase.

Size of Disperse Phase

The size of the disperse phase also can have a significant effect onscattering. If the disperse phase particles are too small (i.e., lessthan about 1/30 the wavelength of light in the medium of interest) andif there are many particles per cubic wavelength, the optical bodybehaves as a medium with an effective index of refraction somewhatbetween the indices of the two phases along any given axis. In such acase, very little light is scattered. If the particles are too large,the light is specularly reflected from the surface of the particle, withvery little diffusion into other directions. When the particles are toolarge in at least two orthogonal directions, undesirable iridescenceeffects can also occur. Practical limits may also be reached whenparticles become large in that the thickness of the optical body becomesgreater and desirable mechanical properties are compromised.

The dimensions of the particles of the disperse phase after alignmentcan vary depending on the desired use of the optical material. Thus, forexample, the dimensions of the particles may vary depending on thewavelength of electromagnetic radiation that is of interest in aparticular application, with different dimensions required forreflecting or transmitting visible, ultraviolet, infrared, and microwaveradiation. Generally, however, the length of the particles should besuch that they are approximately greater than the wavelength ofelectromagnetic radiation of interest in the medium, divided by 30.

Preferably, in applications where the optical body is to be used as alow loss reflective polarizer, the particles will have a length that isgreater than about 2 times the wavelength of the electromagneticradiation over the wavelength range of interest, and preferably over 4times the wavelength. The average diameter of the particles ispreferably equal or less than the wavelength of the electromagneticradiation over the wavelength range of interest, and preferably lessthan 0.5 of the desired wavelength. While the dimensions of the dispersephase are a secondary consideration in most applications, they become ofgreater importance in thin film applications, where there iscomparatively little diffuse reflection.

Geometry of Disperse Phase

While the index mismatch is the predominant factor relied upon topromote scattering in the films of the present invention (i.e., adiffuse mirror or polarizer made in accordance with the presentinvention has a substantial mismatch in the indices of refraction of thecontinuous and disperse phases along at least one axis), the geometry ofthe particles of the disperse phase can have a secondary effect onscattering. Thus, the depolarization factors of the particles for theelectric field in the index of refraction match and mismatch directionscan reduce or enhance the amount of scattering in a given direction. Forexample, when the disperse phase is elliptical in a cross-section takenalong a plane perpendicular to the axis of orientation, the ellipticalcross-sectional shape of the disperse phase contributes to theasymmetric diffusion in both back scattered light and forward scatteredlight. The effect can either add or detract from the amount ofscattering from the index mismatch, but generally has a small influenceon scattering in the preferred range of properties in the presentinvention.

The shape of the disperse phase particles can also influence the degreeof diffusion of light scattered from the particles. This shape effect isgenerally small but increases as the aspect ratio of the geometricalcross-section of the particle in the plane perpendicular to thedirection of incidence of the light increases and as the particles getrelatively larger. In general, in the operation of this invention, thedisperse phase particles should be sized less than several wavelengthsof light in one or two mutually orthogonal dimensions if diffuse, ratherthan specular, reflection is preferred.

Preferably, for a low loss reflective polarizer, the preferredembodiment consists of a disperse phase disposed within the continuousphase as a series of rod-like structures which, as a consequence oforientation, have a high aspect ratio which can enhance reflection forpolarizations parallel to the orientation direction by increasing thescattering strength and dispersion for that polarization relative topolarizations perpendicular to the orientation direction. However, asindicated in FIGS. 3a-e, the disperse phase may be provided with manydifferent geometries. Thus, the disperse phase may be disk-shaped orelongated disk-shaped, as in FIGS. 3a-c, rod-shaped, as in FIG. 3d-e, orspherical. Other embodiments are contemplated wherein the disperse phasehas cross sections which are approximately elliptical (includingcircular), polygonal, irregular, or a combination of one or more ofthese shapes. The cross-sectional shape and size of the particles of thedisperse phase may also vary from one particle to another, or from oneregion of the film to another (i.e., from the surface to the core).

In some embodiments, the disperse phase may have a core and shellconstruction, wherein the core and shell are made out of the same ordifferent materials, or wherein the core is hollow. Thus, for example,the disperse phase may consist of hollow fibers of equal or randomlengths, and of uniform or non-uniform cross section. The interior spaceof the fibers may be empty, or may be occupied by a suitable mediumwhich may be a solid, liquid, or gas, and may be organic or inorganic.The refractive index of the medium may be chosen in consideration of therefractive indices of the disperse phase and the continuous phase so asto achieve a desired optical effect (i.e., reflection or polarizationalong a given axis).

The geometry of the disperse phase may be arrived at through suitableorientation or processing of the optical material, through the use ofparticles having a particular geometry, or through a combination of thetwo. Thus, for example, a disperse phase having a substantially rod-likestructure can be produced by orienting a film consisting ofapproximately spherical disperse phase particles along a single axis.The rod-like structures can be given an elliptical cross-section byorienting the film in a second direction perpendicular to the first. Asa further example, a disperse phase having a substantially rod-likestructure in which the rods are rectangular in cross-section can beproduced by orienting in a single direction a film having a dispersephase consisting of a series of essentially rectangular flakes.

Stretching is one convenient manner for arriving at a desired geometry,since stretching can also be used to induce a difference in indices ofrefraction within the material. As indicated above, the orientation offilms in accordance with the invention may be in more than onedirection, and may be sequential or simultaneous.

In another example, the components of the continuous and disperse phasesmay be extruded such that the disperse phase is rod-like in one axis inthe unoriented film. Rods with a high aspect ratio may be generated byorienting in the direction of the major axis of the rods in the extrudedfilm. Plate-like structures may be generated by orienting in anorthogonal direction to the major axis of the rods in the extruded film.

The structure in FIG. 2 can be produced by asymmetric biaxialorientation of a blend of essentially spherical particles within acontinuous matrix. Alternatively, the structure may be obtained byincorporating a plurality of fibrous structures into the matrixmaterial, aligning the structures along a single axis, and orienting themixture in a direction transverse to that axis. Still another method forobtaining this structure is by controlling the relative viscosities,shear, or surface tension of the components of a polymer blend so as togive rise to a fibrous disperse phase when the blend is extruded into afilm. In general, it is found that the best results are obtained whenthe shear is applied in the direction of extrusion.

Dimensional Alignment of Disperse Phase

Dimensional alignment is also found to have an effect on the scatteringbehavior of the disperse phase. In particular, it has been observed inoptical bodies made in accordance with the present invention thataligned scatterers will not scatter light symmetrically about thedirections of specular transmission or reflection as randomly alignedscatterers would. In particular, inclusions that have been. elongatedthrough orientation to resemble rods scatter light primarily along (ornear) the surface of a cone centered on the orientation direction andalong the specularly transmitted direction. This may result in ananisotropic distribution of scattered light about the specularreflection and specular transmission directions. For example, for lightincident on such an elongated rod in a direction perpendicular to theorientation direction, the scattered light appears as a band of light inthe plane perpendicular to the orientation direction with an intensitythat decreases with increasing angle away from the specular directions.By tailoring the geometry of the inclusions, some control over thedistribution of scattered light can be achieved both in the transmissivehemisphere and in the reflective hemisphere.

Dimensions of Disperse Phase

In applications where the optical body is to be used as a low lossreflective polarizer, the structures of the disperse phase preferablyhave a high aspect ratio, i.e., the structures are substantially largerin one dimension than in any other dimension. The aspect ratio ispreferably at least 2, and more preferably at least 5. The largestdimension (i.e., the length) is preferably at least 2 times thewavelength of the electromagnetic radiation over the wavelength range ofinterest, and more preferably at least 4 times the desired wavelength.On the other hand, the smaller (i.e., cross-sectional) dimensions of thestructures of the disperse phase are preferably less than or equal tothe wavelength of interest, and more preferably less than 0.5 times thewavelength of interest.

Volume Fraction of Disperse Phase

The volume fraction of the disperse phase also affects the scattering oflight in the optical bodies of the present invention. Within certainlimits, increasing the volume fraction of the disperse phase tends toincrease the amount of scattering that a light ray experiences afterentering the body for both the match and mismatch directions ofpolarized light. This factor is important for controlling the reflectionand transmission properties for a given application. However, if thevolume fraction of the disperse phase becomes too large, lightscattering can diminish. Without wishing to be bound by theory, thisappears to be due to the fact that the disperse phase particles arecloser together, in terms of the wavelength of light, so that theparticles tend to act together as a smaller number of large effectiveparticles.

The desired volume fraction of the disperse phase will depend on manyfactors, including the specific choice of materials for the continuousand disperse phase. However, the volume fraction of the disperse phasewill typically be at least about 1% by volume relative to the continuousphase, more preferably within the range of about 5 to about 15%, andmost preferably within the range of about 15 to about 30%.

Thickness of Optical Body

The thickness of the optical body is also an important parameter whichcan be manipulated to affect reflection and transmission properties inthe present invention. As the thickness of the optical body increases,diffuse reflection also increases, and transmission, both specular anddiffuse, decreases. Thus, while the thickness of the optical body willtypically be chosen to achieve a desired degree of mechanical strengthin the finished product, it can also be used to directly to controlreflection and transmission properties.

Thickness can also be utilized to make final adjustments in reflectionand transmission properties of the optical body. Thus, for example, infilm applications, the device used to extrude the film can be controlledby a downstream optical device which measures transmission andreflection values in the extruded film, and which varies the thicknessof the film (i.e., by adjusting extrusion rates or changing castingwheel speeds) so as to maintain the reflection and transmission valueswithin a predetermined range.

Materials for Continuous/Disperse Phases

Many different materials may be used as the continuous or dispersephases in the optical bodies of the present invention, depending on thespecific application to which the optical body is directed. Suchmaterials include inorganic materials such as silica-based polymers,organic materials such as liquid crystals, and polymeric materials,including monomers, copolymers, grafted polymers, and mixtures or blendsthereof The exact choice of materials for a given application will bedriven by the desired match and mismatch obtainable in the refractiveindices of the continuous and disperse phases along a particular axis,as well as the desired physical properties in the resulting product.However, the materials of the continuous phase will generally becharacterized by being substantially transparent in the region of thespectrum desired.

A further consideration in the choice of materials is that the resultingproduct must contain at least two distinct phases. This may beaccomplished by casting the optical material from two or more materialswhich are immiscible with each other. Alternatively, if it is desired tomake an optical material with a first and second material which are notimmiscible with each other, and if the first material has a highermelting point than the second material, in some cases it may be possibleto embed particles of appropriate dimensions of the first materialwithin a molten matrix of the second material at a temperature below themelting point of the first material. The resulting mixture can then becast into a film, with or without subsequent orientation, to produce anoptical device.

Suitable polymeric materials for use as the continuous or disperse phasein the present invention may be amorphous, semicrystalline, orcrystalline polymeric materials, including materials made from monomersbased on carboxylic acids such as isophthalic, azelaic, adipic, sebacic,dibenzoic, terephthalic, 2,7-naphthalene dicarboxylic, 2,6-naphthalenedicarboxylic, cyclohexanedicarboxylic, and bibenzoic acids (including4,4'-bibenzoic acid), or materials made from the corresponding esters ofthe aforementioned acids (i.e., dimethylterephthalate). Of these,2,6-polyethylene naphthalate (PEN) is especially preferred because ofits strain induced birefringence, and because of its ability to remainpermanently birefringent after birefringence, and because of its abilityto remain permanently birefringent after stretching. PEN has arefractive index for polarized incident light of 550 nm wavelength whichincreases after stretching when the plane of polarization is parallel tothe axis of stretch from about 1.64 to as high as about 1.9, while therefractive index decreases for light polarized perpendicular to the axisof stretch. PEN exhibits a birefringence (in this case, the differencebetween the index of refraction along the stretch direction and theindex perpendicular to the stretch direction) of 0.25 to 0.40 in thevisible spectrum. The birefringence can be increased by increasing themolecular orientation. PEN may be substantially heat stable from about155° C. up to about 230° C., depending upon the processing conditionsutilized during the manufacture of the film.

Polybutylene naphthalate is also a suitable material as well as othercrystalline naphthalene dicarboxylic polyesters. The crystallinenaphthalene dicarboxylic polyesters exhibit a difference in refractiveindices associated with different in-plane axes of at least 0.05 andpreferably above 0.20.

When PEN is used as one phase in the optical material of the presentinvention, the other phase is preferably polymethylmethacrylate (PMMA)or a syndiotactic vinyl aromatic polymer such as polystyrene (sPS).Other preferred polymers for use with PEN are based on terephthalic,isophthalic, sebacic, azelaic or cyclohexanedicarboxylic acid or therelated alkyl esters of these materials. Naphthalene dicarboxylic acidmay also be employed in minor amounts to improve adhesion between thephases. The diol component may be ethylene glycol or a related diol.Preferably, the index of refraction of the selected polymer is less thanabout 1.65, and more preferably, less than about 1.55, although asimilar result may be obtainable by using a polymer having a higherindex of refraction if the same index difference is achieved.

Syndiotactic-vinyl aromatic polymers useful in the current inventioninclude poly(styrene), poly(alkyl styrene), poly(styrene halide),poly(alkyl styrene), poly(vinyl ester benzoate), and these hydrogenatedpolymers and mixtures, or copolymers containing these structural units.Examples of poly(alkyl styrenes) include: poly(methyl styrene),poly(ethyl styrene), poly(propyl styrene), poly(butyl styrene),poly(phenyl styrene), poly(vinyl naphthalene), poly(vinylstyrene), andpoly(acenaphthalene) may be mentioned. As for the poly(styrene halides),examples include: poly(chlorostyrene), poly(bromostyrene), andpoly(fluorostyrene). Examples of poly(alkoxy styrene) include:poly(methoxy styrene), and poly(ethoxy styrene). Among these examples,as particularly preferable styrene group polymers, are: polystyrene,poly(p-methyl styrene), poly(m-methyl styrene), poly(p-tertiary butylstyrene), poly(p-chlorostyrene), poly(m-chloro styrene), poly(p-fluorostyrene), and copolymers of styrene and p-methyl styrene may bementioned.

Furthermore, as comonomers of syndiotactic vinyl-aromatic groupcopolymers, besides monomers of above explained styrene group polymer,olefin monomers such as ethylene, propylene, butene, hexene, or octene;diene monomers such as butadiene, isoprene; polar vinyl monomers such ascyclic diene monomer, methyl methacrylate, maleic acid anhydride, oracrylonitrile may be mentioned.

The syndiotactic-vinyl aromatic polymers of the present invention may beblock copolymers, random copolymers, or alternating copolymers.

The vinyl aromatic polymer having high level syndiotactic structurereferred to in this invention generally includes polystyrene havingsyndiotacticity of higher than 75% or more, as determined by carbon-13nuclear magnetic resonance. Preferably, the degree of syndiotacticity ishigher than 85% racemic diad, or higher than 30%, or more preferably,higher than 50%, racemic pentad.

In addition, although there are no particular restrictions regarding themolecular weight of this syndiotactic-vinyl aromatic group polymer,preferably, the weight average molecular weight is greater than 10,000and less than 1,000,000, and more preferably, greater than 50,000 andless than 800,000.

As for said other resins, various types may be mentioned, including, forinstance, vinyl aromatic group polymers with atactic structures, vinylaromatic group polymers with isotactic structures, and all polymers thatare miscible. For example, polyphenylene ethers show good miscibilitywith the previous explained vinyl aromatic group polymers. Furthermore,the composition of these miscible resin components is preferably between70 to 1 weight %, or more preferably, 50 to 2 weight %. When compositionof miscible resin component exceeds 70 weight %, degradation on the heatresistance may occur, and is usually not desirable.

It is not required that the selected polymer for a particular phase be acopolyester or copolycarbonate. Vinyl polymers and copolymers made frommonomers such as vinyl naphthalenes, styrenes, ethylene, maleicanhydride, acrylates, and methacrylates may also be employed.Condensation polymers, other than polyesters and polycarbonates, canalso be utilized. Suitable condensation polymers include polysulfones,polyamides, polyurethanes, polyamic acids, and polyimides. Naphthalenegroups and halogens such as chlorine, bromine and iodine are useful inincreasing the refractive index of the selected polymer to the desiredlevel (1.59 to 1.69) if needed to substantially match the refractiveindex if PEN is the host. Acrylate groups and fluorine are particularlyuseful in decreasing the refractive index.

Minor amounts of comonomers may be substituted into the naphthalenedicarboxylic acid polyester so long as the large refractive indexdifference in the orientation direction(s) is not substantiallycompromised. A smaller index difference (and therefore decreasedreflectivity) may be counterbalanced by advantages in any of thefollowing: improved adhesion between the continuous and disperse phase,lowered temperature of extrusion, and better match of melt viscosities.

Region of Spectrum

While the present invention is frequently described herein withreference to the visible region of the spectrum, various embodiments ofthe present invention can be used to operate at different wavelengths(and thus frequencies) of electromagnetic radiation through appropriatescaling of the components of the optical body. Thus, as the wavelengthincreases, the linear size of the components of the optical body may beincreased so that the dimensions of these components, measured in unitsof wavelength, remain approximately constant.

Of course, one major effect of changing wavelength is that, for mostmaterials of interest, the index of refraction and the absorptioncoefficient change. However, the principles of index match and mismatchstill apply at each wavelength of interest, and may be utilized in theselection of materials for an optical device that will operate over aspecific region of the spectrum. Thus, for example, proper scaling ofdimensions will allow operation in the infrared, near-ultraviolet, andultra-violet regions of the spectrum. In these cases, the indices ofrefraction refer to the values at these wavelengths of operation, andthe body thickness and size of the disperse phase scattering componentsshould also be approximately scaled with wavelength. Even more of theelectromagnetic spectrum can be used, including very high, ultrahigh,microwave and millimeter wave frequencies. Polarizing and diffusingeffects will be present with proper scaling to wavelength and theindices of refraction can be obtained from the square root of thedielectric function (including real and imaginary parts). Usefulproducts in these longer wavelength bands can be diffuse reflectivepolarizers and partial polarizers.

In some embodiments of the present invention, the optical properties ofthe optical body vary across the wavelength band of interest. In theseembodiments, materials may be utilized for the continuous and/ordisperse phases whose indices of refraction, along one or more axes,varies from one wavelength region to another. The choice of continuousand disperse phase materials, and the optical properties (i.e., diffuseand disperse reflection or specular transmission) resulting from aspecific choice of materials, will depend on the wavelength band ofinterest.

Skin Layers

A layer of material which is substantially free of a disperse phase maybe coextensively disposed on one or both major surfaces of the film,i.e., the extruded blend of the disperse phase and the continuous phase.The composition of the layer, also called a skin layer, may be chosen,for example, to protect the integrity of the disperse phase within theextruded blend, to add mechanical or physical properties to the finalfilm or to add optical functionality to the final film. Suitablematerials of choice may include the material of the continuous phase orthe material of the disperse phase. Other materials with a meltviscosity similar to the extruded blend may also be useful.

A skin layer or layers may reduce the wide range of shear intensitiesthe extruded blend might experience within the extrusion process,particularly at the die. A high shear environment may cause undesirablesurface voiding and may result in a textured surface. A broad range ofshear values throughout the thickness of the film may also prevent thedisperse phase from forming the desired particle size in the blend.

A skin layer or layers may also add physical strength to the resultingcomposite or reduce problems during processing, such as, for example,reducing the tendency for the film to split during the orientationprocess. Skin layer materials which remain amorphous may tend to makefilms with a higher toughness, while skin layer materials which aresemicrystalline may tend to make films with a higher tensile modulus.Other functional components such as antistatic additives, UV absorbers,dyes, antioxidants, and pigments, may be added to the skin layer,provided they do not substantially interfere with the desired opticalproperties of the resulting product.

The skin layers may be applied to one or two sides of the extruded blendat some point during the extrusion process, i.e., before the extrudedblend and skin layer(s) exit the extrusion die. This may be accomplishedusing conventional coextrusion technology, which may include using athree-layer coextrusion die. Lamination of skin layer(s) to a previouslyformed film of an extruded blend is also possible. Total skin layerthicknesses may range from about 2% to about 50% of the total blend/skinlayer thickness.

A wide range of polymers are suitable for skin layers. Predominantlyamorphous polymers include copolyesters based on one or more ofterephthalic acid, 2,6-naphthalene dicarboxylic acid, isophthalic acidphthalic acid, or their alkyl ester counterparts, and alkylene diols,such as ethylene glycol. Examples of semicrystalline polymers are2,6-polyethylene naphthalate, polyethylene terephthalate, and nylonmaterials.

Antireflection Layers

The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers. Suchlayers, which may or may not be polarization sensitive, serve toincrease transmission and to reduce reflective glare. An anti-reflectivelayer may be imparted to the films and optical devices of the presentinvention through appropriate surface treatment, such as coating orsputter etching.

In some embodiments of the present invention, it is desired to maximizethe transmission and/or minimize the specular reflection for certainpolarizations of light. In these embodiments, the optical body maycomprise two or more layers in which at least one layer comprises ananti-reflection system in close contact with a layer providing thecontinuous and disperse phases. Such an anti-reflection system acts toreduce the specular reflection of the incident light and to increase theamount of incident light that enters the portion of the body comprisingthe continuous and disperse layers. Such a function can be accomplishedby a variety of means well known in the art. Examples are quarter waveanti-reflection layers, two or more layer anti-reflective stack, gradedindex layers, and graded density layers. Such anti-reflection functionscan also be used on the transmitted light side of the body to increasetransmitted light if desired.

Microvoiding

In some embodiments, the materials of the continuous and disperse phasesmay be chosen so that the interface between the two phases will besufficiently weak to result in voiding when the film is oriented. Theaverage dimensions of the voids may be controlled through carefulmanipulation of processing parameters and stretch ratios, or throughselective use of compatibilizers. The voids may be back-filled in thefinished product with a liquid, gas, or solid. Voiding may be used inconjunction with the aspect ratios and refractive indices of thedisperse and continuous phases to produce desirable optical propertiesin the resulting film.

More Than Two Phases

The optical bodies made in accordance with the present invention mayalso consist of more than two phases. Thus, for example, an opticalmaterial made in accordance with the present invention can consist oftwo different disperse phases within the continuous phase. The seconddisperse phase could be randomly or non-randomly dispersed throughoutthe continuous phase, and can be randomly aligned or aligned along acommon axis.

Optical bodies made in accordance with the present invention may alsoconsist of more than one continuous phase. Thus, in some embodiments,the optical body may include, in addition to a first continuous phaseand a disperse phase, a second phase which is co-continuous in at leastone dimension with the first continuous phase. In one particularembodiment, the second continuous phase is a porous, sponge-likematerial which is coextensive with the first continuous phase (i.e., thefirst continuous phase extends through a network of channels or spacesextending through the second continuous phase, much as water extendsthrough a network of channels in a wet sponge). In a related embodiment,the second continuous phase is in the form of a dendritic structurewhich is coextensive in at least one dimension with the first continuousphase.

Multilayer Combinations

If desired, one or more sheets of a continuous/disperse phase film madein accordance with the present invention may be used in combinationwith, or as a component in, a multilayered film (i.e., to increasereflectivity). Suitable multilayered films include those of the typedescribed in WO 95/17303 (Ouderkirk et al.). In such a construction, theindividual sheets may be laminated or otherwise adhered together or maybe spaced apart. If the optical thicknesses of the phases within thesheets are substantially equal (that is, if the two sheets present asubstantially equal and large number of scatterers to incident lightalong a given axis), the composite will reflect, at somewhat greaterefficiency, substantially the same band width and spectral range ofreflectivity (i.e., "band") as the individual sheets. If the opticalthicknesses of phases within the sheets are not substantially equal, thecomposite will reflect across a broader band width than the individualphases. A composite combining mirror sheets with polarizer sheets isuseful for increasing total reflectance while still polarizingtransmitted light. Alternatively, a single sheet may be asymmetricallyand biaxially oriented to produce a film having selective reflective andpolarizing properties.

FIG. 5 illustrates one example of this embodiment of the presentinvention. There, the optical body consists of a multilayer film 20 inwhich the layers alternate between layers of PEN 22 and layers of co-PEN24. Each PEN layer includes a disperse phase of syndiotactic polystyrene(sPS) within a matrix of PEN. This type of construction is desirable inthat it promotes lower off-angle color. Furthermore, since the layeringor inclusion of scatterers averages out light leakage, control overlayer thickness is less critical, allowing the film to be more tolerableof variations in processing parameters.

Any of the materials previously noted may be used as any of the layersin this embodiment, or as the continuous or disperse phase within aparticular layer. However, PEN and co-PEN are particularly desirable asthe major components of adjacent layers, since these materials promotegood laminar adhesion.

Also, a number of variations are possible in the arrangement of thelayers. Thus, for example, the layers can be made to follow a repeatingsequence through part or all of the structure. One example of this is aconstruction having the layer pattern . . . ABCABC . . . , wherein A, B,and C are distinct materials or distinct blends or mixtures of the sameor different materials, and wherein one or more of A, B, or C containsat least one disperse phase and at least one continuous phase. The skinlayers are preferably the same or chemically similar materials.

Additives

The optical materials of the present invention may also comprise othermaterials or additives as are known to the art. Such materials includepigments, dyes, binders, coatings, fillers, compatibilizers,antioxidants (including sterically hindered phenols), surfactants,antimicrobial agents, antistatic agents, flame retardants, foamingagents, lubricants, reinforcers, light stabilizers (including UVstabilizers or blockers), heat stabilizers, impact modifiers,plasticizers, viscosity modifiers, and other such materials.Furthermore, the films and other optical devices made in accordance withthe present invention may include one or more outer layers which serveto protect the device from abrasion, impact, or other damage, or whichenhance the processability or durability of the device.

Suitable lubricants for use in the present invention include calciumsterate, zinc sterate, copper sterate, cobalt sterate, molybdenumneodocanoate, and ruthenium (III) acetylacetonate.

Antioxidants useful in the present invention include4,4'-thiobis-(6-t-butyl-m-cresol),2,2'-methylenebis-(4-methyl-6-t-butyl-butylphenol),octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate,bis-(2,4-di-t-butylphenyl)pentaerythritol diphosphite, Irganox™ 1093(1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl) -dioctadecylester phosphonic acid), Irganox™ 1098(N,N'-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamide),Naugaard™ 445 (aryl amine), Irganox™ L 57 (alkylated diphenylamine),Irganox™ L 115 (sulfur containing bisphenol), Irganox™ LO 6 (alkylatedphenyl-delta-napthylamine), Ethanox 398 (flourophosphonite), and2,2'-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite.

A group of antioxidants that are especially preferred are stericallyhindered phenols, including butylated hydroxytoluene (BHT), Vitamin E(di-alpha-tocopherol), Irganox™ 1425WL(calciumbis-(O-ethyl(3,5-di-t-butyl-4-hydroxybenzyl))phosphonate), Irganox™ 1010(tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane),Irganox™ 1076 (octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate),Ethanox™ 702 (hindered bis phenolic), Etanox 330 (high molecular weighthindered phenolic), and Ethanox™ 703 (hindered phenolic amine).

Dichroic dyes are a particularly useful additive in some applications towhich the optical materials of the present invention may be directed,due to their ability to absorb light of a particular polarization whenthey are molecularly aligned within the material. When used in a film orother material which predominantly scatters only one polarization oflight, the dichroic dye causes the material to absorb one polarizationof light more than another. Suitable dichroic dyes for use in thepresent invention include Congo Red (sodium diphenyl-bis-α-naphthylaminesulfonate), methylene blue, stilbene dye (Color Index (CI)=620), and1,1'-diethyl-2,2'-cyanine chloride (CI=374 (orange) or CI=518 (blue)).The properties of these dyes, and methods of making them, are describedin E.H. Land, Colloid Chemistry (1946). These dyes have noticeabledichroism in polyvinyl alcohol and a lesser dichroism in cellulose. Aslight dichroism is observed with Congo Red in PEN.

Other suitable dyes include the following materials: ##STR1## Theproperties of these dyes, and methods of making them, are discussed inthe Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661(4th Ed. 1993), and in the references cited therein.

When a dichroic dye is used in the optical bodies of the presentinvention, it may be incorporated into either the continuous or dispersephase. However, it is preferred that the dichroic dye is incorporatedinto the disperse phase.

Dychroic dyes in combination with certain polymer systems exhibit theability to polarize light to varying degrees. Polyvinyl alcohol andcertain dichroic dyes may be used to make films with the ability topolarize light. Other polymers, such as polyethylene terephthalate orpolyamides, such as nylon-6, do not exhibit as strong an ability topolarize light when combined with a dichroic dye. The polyvinyl alcoholand dichroic dye combination is said to have a higher dichroism ratiothan, for example, the same dye in other film forming polymer systems. Ahigher dichroism ratio indicates a higher ability to polarize light.

Molecular alignment of a dichroic dye within an optical body made inaccordance with the present invention is preferably accomplished bystretching the optical body after the dye has been incorporated into it.However, other methods may also be used to achieve molecular alignment.Thus, in one method, the dichroic dye is crystallized, as throughsublimation or by crystallization from solution, into a series ofelongated notches that are cut, etched, or otherwise formed in thesurface of a film or other optical body, either before or after theoptical body has been oriented. The treated surface may then be coatedwith one or more surface layers, may be incorporated into a polymermatrix or used in a multilayer structure, or may be utilized as acomponent of another optical body. The notches may be created inaccordance with a predetermined pattern or diagram, and with apredetermined amount of spacing between the notches, so as to achievedesirable optical properties.

In a related embodiment, the dichroic dye may be disposed within one ormore hollow fibers or other conduits, either before or after the hollowfibers or conduits are disposed within the optical body. The hollowfibers or conduits may be constructed out of a material that is the sameor different from the surrounding material of the optical body.

In yet another embodiment, the dichroic dye is disposed along the layerinterface of a multilayer construction, as by sublimation onto thesurface of a layer before it is incorporated into the multilayerconstruction. In still other embodiments, the dichroic dye is used to atleast partially backfill the voids in a microvoided film made inaccordance with the present invention.

Applications of Present Invention

The optical bodies of the present invention are particularly useful asdiffuse polarizers. However, optical bodies may also be made inaccordance with the invention which operate as reflective polarizers ordiffuse mirrors. In these applications, the construction of the opticalmaterial is similar to that in the diffuser applications describedabove. However, these reflectors will generally have a much largerdifference in the index of refraction along at least one axis. Thisindex difference is typically at least about 0.1, more preferably about0.15, and most preferably about 0.2.

Reflective polarizers have a refractive index difference along one axis,and substantially matched indices along another. Reflective films, onthe other hand, differ in refractive index along at least two in-filmplane orthogonal axes. However, the reflective properties of theseembodiments need not be attained solely by reliance on refractive indexmismatches. Thus, for example, the thickness of the films could beadjusted to attain a desired degree of reflection. In some cases,adjustment of the thickness of the film may cause the film to go frombeing a transmissive diffuser to a diffuse reflector.

The reflective polarizer of the present invention has many differentapplications, and is particularly useful in liquid crystal displaypanels. In addition, the polarizer can be constructed out of PEN orsimilar materials which are good ultraviolet filters and which absorbultraviolet light efficiently up to the edge of the visible spectrum.The reflective polarizer can also be used as a thin infrared sheetpolarizer.

Overview of Examples

The following Examples illustrate the production of various opticalmaterials in accordance with the present invention, as well as thespectral properties of these materials. Unless otherwise indicated,percent composition refers to percent composition by weight. Thepolyethylene naphthalate resin used was produced for these samples usingethylene glycol and dimethyl-2,6-naphthalenedicarboxylate, availablefrom Amoco Corp., Chicago, Ill. These reagents were polymerized tovarious intrinsic viscosities (IV) using conventional polyester resinpolymerization techniques. Syndiotactic polystyrene (sPS) may beproduced in accordance with the method disclosed in U.S. Pat. No.4,680,353 (Ishihara et al). The examples includes various polymer pairs,various fractions of continuous and disperse phases and other additivesor process changes as discussed below.

Stretching or orienting of the samples was provided using eitherconventional orientation equipment used for making polyester film or alaboratory batch orienter. The laboratory batch orienter used wasdesigned to use a small piece of cast material (7.5 cm by 7.5 cm) cutfrom the extruded cast web and held by a square array of 24 grippers (6on each side). The orientation temperature of the sample was controlleda hot air blower and the film sample was oriented through a mechanicalsystem that increased the distance between the grippers in one or bothdirections at a controlled rate. Samples stretched in both directionscould be oriented sequentially or simultaneously. For samples that wereoriented in the constrained mode (C), all grippers hold the web and thegrippers move only in one dimension. Whereas, in the unconstrained mode(U), the grippers that hold the film at a fixed dimension perpendicularto the direction of stretch are not engaged and the film is allowed torelax or neckdown in that dimension.

Polarized diffuse transmission and reflection were measured using aPerkin Elmer Lambda 19 ultraviolet/visible/near infraredspectrophotometer equipped with a Perkin Elmer Labsphere S900-1000 150millimeter integrating sphere accessory and a Glan-Thompson cubepolarizer. Parallel and crossed transmission and reflection values weremeasured with the e-vector of the polarized light parallel orperpendicular, respectively, to the stretch direction of the film. Allscans were continuous and were conducted with a scan rate of 480nanometers per minute and a slit width of 2 nanometers. Reflection wasperformed in the "V-reflection" mode. Transmission and reflectancevalues are averages of all wavelengths from 400 to 700 nanometers.

EXAMPLE 1

In Example 1, an optical film was made in accordance with the inventionby extruding a blend of 75% polyethylene naphthalate (PEN) as thecontinuous or major phase and 25% of polymethylmethacrylate (PMMA) asthe disperse or minor phase into a cast film or sheet about 380 micronsthick using conventional extrusion and casting techniques. The PEN hadan intrinsic viscosity (IV) of 0.52 (measured in 60% phenol, 40%dichlorobenzene). The PMMA was obtained from ICI Americas, Inc.,Wilmington, Del., under the product designation CP82. The extruder usedwas a 3.15 cm (1.24") Brabender with a 1 tube 60 μm Tegra filter. Thedie was a 30.4 cm (12") EDI Ultraflex™ 40.

About 24 hours after the film was extruded, the cast film was orientedin the width or transverse direction (TD) on a polyester film tenteringdevice. The stretching was accomplished at about 9.1 meters per minute(30 ft/min) with an output width of about 140 cm (55 inches) and astretching temperature of about 160° C. (320° F.). The totalreflectivity of the stretched sample was measured with an integratingsphere attachment on a Lambda 19 spectrophotometer with the sample beampolarized with a Glan-Thompson cube polarizer. The sample had a 75%parallel reflectivity (i.e., reflectivity was measured with the stretchdirection of the film parallel to the e-vector of the polarized light),and 52% crossed reflectivity (i.e., reflectivity was measured with thee-vector of the polarized light perpendicular to the stretch direction).

EXAMPLE 2

In Example 2, an optical film was made and evaluated in a manner similarto Example 1 except using a blend of 75% PEN, 25% syndiotacticpolystyrene (sPS), 0.2% of a polystyrene glycidyl methacrylatecompatibilizer, and 0.25% each of Irganox™ 1010 and Ultranox™ 626. Thesynthesis of polystyrene glycidyl methacrylate is described in PolymerProcesses, "Chemical Technology of Plastics, Resins, Rubbers, Adhesivesand Fibers", Vol. 10, Chap. 3, pp. 69-109 (1956)(Ed. by Calvin E.Schildknecht).

The PEN had an intrinsic viscosity of 0.52 measured in 60% phenol, 40%dichlorobenzene. The sPS was obtained from Dow Chemical Co. and had aweight average molecular weight of about 200,000, designatedsubsequently as sPS-200-0. The parallel reflectivity on the stretchedfilm sample was determined to be 73.3%, and the crossed reflectivity wasdetermined to be 35%.

EXAMPLE 3

In Example 3, an optical film was made and evaluated in a manner similarto Example 2 except the compatibilizer level was raised to 0.6%. Theresulting parallel reflectivity was determined to be 81% and the crossedreflectivity was determined to be 35.6%.

EXAMPLE 4

In Example 4, an three layer optical film was made in accordance withthe present invention utilizing conventional three layer coextrusiontechniques. The film had a core layer and a skin layer on each side ofthe core layer. The core layer consisted of a blend of 75% PEN and 25%sPS 200-4 (the designation sPS-200-4 refers to a copolymer ofsyndiotactic-polystyrene containing 4 mole % of para-methyl styrene),and each skin layer consisted of 100% PEN having an intrinsic viscosityof 0.56 measured in 60% phenol, 40% dichlorobenzene.

The resulting three-layer cast film had a core layer thickness of about415 microns, and each skin layer was about 110 microns thick for a totalthickness of about 635 microns. A laboratory batch stretcher was used tostretch the resulting three-layer cast film about 6 to 1 in the machinedirection (MD) at a temperature of about 129° C. Because the edges ofthe film sample parallel to the stretch direction were not gripped bythe lab stretcher, the sample was unconstrained in the transversedirection (TD) and the sample necked-down in the TD about 50% as aresult of the stretch procedure.

Optical performance was evaluated in a manner similar to Example 1. Theparallel reflectivity was determined to be 80.1%, and the crossedreflectivity was determined to be 15%. These results demonstrate thatthe film performs as a low absorbing, energy conserving system.

EXAMPLES 5-29

In Examples 5-29, a series of optical films were produced and evaluatedin a manner similar to Example 4, except the sPS fraction in the corelayer and the IV of the PEN resin used were varied as shown in Table 1.The IV of the PEN resin in the core layer and that in the skin layerswas the same for a given sample. The total thickness of the cast sheetwas about 625 microns with about two-thirds of this total in the corelayer and the balance in the skin layers which were approximately equalin thickness. Various blends of PEN and sPS in the core layer wereproduced, as indicated in Table 1. The films were stretched to a stretchratio of about 6:1 in either the machine direction (MD) or in thetransverse direction (TD) at various temperatures as indicated inTable 1. Some of the samples were constrained (C) in the directionperpendicular to the stretch direction to prevent the sample fromnecking down during stretching. The samples labeled "U" in Table 1 wereunconstrained and permitted to neckdown in the unconstrained dimension.Certain optical properties of the stretched samples, including percenttransmission, reflection, and absorption, were measured along axes bothparallel and crossed or perpendicular to the direction of stretch. Theresults are summarized in TABLE 1.

Heat setting, as indicated for Examples 24-27, was accomplished bymanually constraining the two edges of the stretched sample which wereperpendicular to the direction of stretch by clamping to anappropriately sized rigid frame and placing the clamped sample in anoven at the indicated temperature for 1 minute. The two sides of thesample parallel to the direction of stretch were unconstrained (U) ornot clamped and allowed to neckdown. The heatsetting of Example 29 wassimilar except all four of the edges of the stretched sample wereconstrained (C) or clamped. Example 28 was not heat set.

                                      TABLE 1                                     __________________________________________________________________________                  Stretch                                                              Stretch                                                                           Stretch                                                                            Cons-           Cons-                                           Example                                                                            Temp.                                                                             Direction                                                                          trained                                                                           PEN                                                                              Fraction                                                                          Heat Set                                                                           trained                                                                            Trans.                                                                            Trans.                                                                            Reflec.                                                                           Reflec.                        Number                                                                             (° C.)                                                                     (MD/TD)                                                                            (C/U)                                                                             IV (sPS)                                                                             Temp.                                                                              Heat Set                                                                           (Perp.)                                                                           (Para.)                                                                           (Perp.)                                                                           (Para.)                        __________________________________________________________________________    5    135 TD   C   0.53                                                                             0.25          76.2                                                                              20.4                                                                              22.6                                                                              75.3                           6    135 TD   C   0.47                                                                             0.75          80.2                                                                              58.4                                                                              19.4                                                                              40                             7    142 TD   C   0.53                                                                             0.25          74.2                                                                              21.8                                                                              25.3                                                                              77.3                           8    142 TD   C   0.47                                                                             0.75          76.0                                                                              41.0                                                                              23.8                                                                              55.6                           9    129 TD   C   0.53                                                                             0.25          71.2                                                                              21.2                                                                              26.5                                                                              76.2                           10   129 TD   C   0.47                                                                             0.75          76.8                                                                              48.9                                                                              22.4                                                                              49.6                           11   129 MD   U   0.53                                                                             0.25          81.5                                                                              27.6                                                                              17.2                                                                              67                             12   129 TD   U   0.53                                                                             0.25          66.8                                                                              22.1                                                                              25  71.9                           13   129 MD   U   0.47                                                                             0.25          79.5                                                                              20.3                                                                              19.3                                                                              73.7                           14   129 TD   U   0.47                                                                             0.25          66.3                                                                              26.2                                                                              32.5                                                                              69.4                           15   129 TD   U   0.47                                                                             0.5           73.0                                                                              26.2                                                                              24.7                                                                              68.7                           16   129 MD   U   0.47                                                                             0.5           75.4                                                                              20.6                                                                              23.2                                                                              76.1                           17   129 MD   U   0.47                                                                             0.1           82.1                                                                              27.3                                                                              16.9                                                                              67                             18   129 MD   U   0.56                                                                             0.25          80.1                                                                              15.0                                                                              18  80.3                           19   129 TD   U   0.56                                                                             0.25          70.2                                                                              21.6                                                                              25.2                                                                              70.7                           20   129 MD   C   0.47                                                                             0.25          75.8                                                                              28.7                                                                              23.4                                                                              70.1                           21   129 MD   C   0.47                                                                             0.5           79.8                                                                              27.8                                                                              19.7                                                                              70.8                           22   135 MD   C   0.47                                                                             0.1           80.5                                                                              36.7                                                                              19.2                                                                              62.6                           23   135 MD   C   0.53                                                                             0.25          77.2                                                                              21.1                                                                              21.8                                                                              76.6                           24   129 MD   U   0.56                                                                             0.25                                                                              150  U    83.7                                                                              17.3                                                                              17.3                                                                              74                             25   129 MD   U   0.56                                                                             0.25                                                                              220  U    82.1                                                                              16  18  75.8                           26   129 MD   U   0.56                                                                             0.25                                                                              135  U    84.7                                                                              17  18  75.3                           27   129 MD   U   0.56                                                                             0.25                                                                              165  U    83  16  16.5                                                                              76.3                           28   129 MD   U   0.56                                                                             0.25                                                                              CNTRL     83.7                                                                              17  17.5                                                                              76                             29   129 MD   U   0.56                                                                             0.25                                                                              230  C                                               29   129 MD   U   0.56                                                                             0.25                                                                              230  C                                               __________________________________________________________________________

All of the above samples were observed to contain varying shapes of thedisperse phase depending on the location of the disperse phase withinthe body of the film sample. The disperse phase inclusions locatednearer the surfaces of the samples were observed to be of an elongatedshape rather than more nearly spherical. The inclusions which are morenearly centered between the surfaces of the samples may be more nearlyspherical. This is true even for the samples with the skin layers, butthe magnitude of the effect is reduced with the skin layers. Theaddition of the skin layers improves the processing of the films byreducing the tendency for splitting during the stretching operation.

Without wishing to be bound by theory, the elongation of the inclusions(disperse phase) in the core layer of the cast film is thought to be theresult of shear on the blend as it is transported through the die. Thiselongation feature may be altered by varying physical dimensions of thedie, extrusion temperatures, flow rate of the extrudate, as well aschemical aspects of the continuous and disperse phase materials whichwould alter their relative melt viscosities. Certain applications oruses may benefit from providing some elongation to the disperse phaseduring extrusion. For those applications which are subsequentlystretched in the machine direction, starting with a disperse phaseelongated during extrusion may allow a higher aspect ratio to be reachedin the resulting disperse phase.

Another notable feature is the fact that a noticeable improvement inperformance is observed when the same sample is stretched unconstrained.Thus, in Example 9, the % transmission was 79.5% and 20.3% in theparallel and perpendicular directions, respectively. By contrast, thetransmission in Example 16 was only 75.8% and 28.7% in the parallel andperpendicular directions, respectively. There is a thickness increaserelative to constrained stretching when samples are stretchedunconstrained, but since both transmission and extinction improve, theindex match is probably being improved.

An alternative way to provide refractive index control is to modify thechemistry of the materials. For example, a copolymer of 30 wt % ofinterpolymerized units derived from terephthalic acid and 70 wt % ofunits derived from 2,6-naphthalic acid has a refractive index 0.02 unitslower than a 100% PEN polymer. Other monomers or ratios may haveslightly different results. This type of change may be used to moreclosely match the refractive indices in one axis while only causing aslight reduction in the axis which desires a large difference. In otherwords, the benefits attained by more closely matching the index valuesin one axis more than compensate for the reduction in an orthogonal axisin which a large difference is desired. Secondly, a chemical change maybe desirable to alter the temperature range in which stretching occurs.A copolymer of sPS and varying ratios of para methyl styrene monomerwill alter the optimum stretch-temperature range. A combination of thesetechniques may be necessary to most effectively optimize the totalsystem for processing and resulting refractive index matches anddifferences. Thus, an improved control of the final performance may beattained by optimizing the process and chemistry in terms of stretchingconditions and further adjusting the chemistry of the materials tomaximize the difference in refractive index in at least one axis andminimizing the difference at least one orthogonal axis.

These samples displayed better optical performance if oriented in the MDrather than TD direction (compare Examples 14-15). Without wishing to bebound by theory, it is believed that different geometry inclusions aredeveloped with an MD orientation than with a TD orientation and thatthese inclusions have higher aspect ratios, making non-ideal end effectsless important. The non-ideal end effects refers to the complexgeometry/index of refraction relationship at the tip of each end of theelongated particles. The interior or non-end of the particles arethought to have a uniform geometry and refractive index which is thoughtto be desirable. Thus, the higher the percentage of the elongatedparticle that is uniform, the better the optical performance.

The extinction ratio of these materials is the ratio of the transmissionfor polarizations perpendicular to the stretch direction to thatparallel to the stretch direction. For the examples cited in Table 1,the extinction ratio ranges between about 2 and about 5, althoughextinction ratios up to 7 have been observed in optical bodies made inaccordance with the present invention. It is expected that even higherextinction ratios can be achieved by adjusting film thickness, inclusionvolume fraction, particle size, and the degree of index match andmismatch.

EXAMPLES 30-100

In Examples 30-100, samples of the invention were made using variousmaterials as listed in Table 2. PEN 42, PEN 47, PEN 53, PEN 56, and PEN60 refer to polyethylene naphthalate having an intrinsic viscosity (IV)of 0.42, 0.47, 0.53, 0.56, and 0.60, respectively, measured in 60%phenol, 40% dichlorobenzene. The particular sPS-200-4 used was obtainedfrom Dow Chemical Co. Ecdel™ 9967 and Eastar™ are copolyesters which areavailable commercially from Eastman Chemical Co., Rochester, N.Y.Surlyn™ 1706 is an ionomer resin available from E.I. du Pont de Nemours& Co., Wilmington, Del. The materials listed as Additive 1 or 2 includepolystyrene glycidyl methacrylate. The designations GMAPS2, GMAPS5, andGMAPS8 refer to glycidyl methacrylate having 2, 5, and 8% by weight,respectively, of glycidyl methacrylate in the total copolymer. ETPBrefers to the crosslinking agent ethyltriphenylphosphonium bromide. PMMAVO44 refers to a polymethylmethacrylate available commercially fromAtohaas North America, Inc.

The optical film samples were produced in a manner similar to Example 4except for the differences noted in Table 2 and discussed below. Thecontinuous phase and its ratio of the total is reported as major phase.The disperse phase and its ratio of the total is reported as minorphase. The value reported for blend thickness represents the approximatethickness of the core layer in microns. The thickness of the skin layersvaried when the core layer thickness varied, but was kept to a constantratio, i.e., the skin layers were approximately equal and the total ofthe two skin layers was about one-third of the total thickness. The sizeof the disperse phase was determined for some samples by either scanningelectron microscope (SEM) or transmission electron microscope (TEM).Those examples which were subsequently stretched using the laboratorybatch orienter are shown by an "X" in the column labeled BatchStretched.

                                      TABLE 2                                     __________________________________________________________________________              Major    Minor                                                      Example                                                                            Major                                                                              Phase                                                                             Minor                                                                              Phase                                                                             Core Layer  Additive TEM  Batch                        Number                                                                             Phase                                                                              (%) Phase                                                                              (%) (microns)                                                                           Additive 1                                                                          2    SEMs                                                                              (microns)                                                                          Stretched                    __________________________________________________________________________    30   PEN.42                                                                             75  sPS-200-4                                                                          25  9.8   --    --   --  --   --                           31   PEN.42                                                                             75  sPS-200-4                                                                          25  16.3  --    --   10  --   --                           32   PEN.47                                                                             75  sPS-200-4                                                                          25  9.8   --    --   --  --   x                            33   PEN.47                                                                             75  sPS-200-4                                                                          25  16.3  --    --   8   --   x                            34   PEN.47                                                                             50  sPS-200-4                                                                          50  9.8   --    --   --  --   --                           35   PEN.47                                                                             50  sPS-200-4                                                                          50  16.3  --    --   5   --   x                            36   PEN.47                                                                             90  sPS-2Q0-4                                                                          10  9.8   --    --   --  --   --                           37   PEN.47                                                                             90  sPS-200-4                                                                          10  16.3  --    --   3   --   x                            38   PEN.53                                                                             75  sPS-200-4                                                                          25  9.8   --    --   --  --   --                           39   PEN.53                                                                             75  sPS-200-4                                                                          25  16.3  --    --   7   --   x                            40   PEN.56                                                                             75  sPS-200-4                                                                          25  9.8   --    --   --  --   --                           41   PEN.56                                                                             75  sPS-200-4                                                                          25  16.3  --    --   6   --   x                            42   sPS-200-                                                                           75  PEN.42                                                                             25  9.8   --    --   --  --   --                                4                                                                        43   sPS-200-                                                                           75  PEN.42                                                                             25  16.3  --    --   --  --   --                                4                                                                        44   sPS-200-                                                                           75  PEN.47                                                                             25  9.8   --    --   --  --   --                                4                                                                        45   sPS-200-                                                                           75  PEN.47                                                                             25  16.3  --    --   --  --   x                                 4                                                                        46   sPS-200-                                                                           75  PEN.53                                                                             25  16.3  --    --   --  --   --                                4                                                                        47   sPS-200-                                                                           75  PEN.53                                                                             25  9.8   --    --   --  --   --                                4                                                                        48   sPS-200-                                                                           75  PEN.56                                                                             25  9.8   --    --   --  --   --                                4                                                                        49   sPS-200-                                                                           75  PEN.56                                                                             25  16.3  --    --   --  --   --                                4                                                                        50   PET.60                                                                             75  Ecdel ™                                                                         25  16.3  --    --   --  --   --                                         9967                                                            51   PET.60                                                                             75  Surlyn ™                                                                        25  16.3  --    --   2   --   --                                         1706                                                            52   PEN.47                                                                             75  Ecdel ™                                                                         25  16.3  --    --   2   --   x                                          9967                                                            53   PEN.47                                                                             100 --   --  16.3  --    --   --  --   --                           54   PEN.47                                                                             75  sPS-200                                                                            25  16.3  --    --   --  --   --                           55   PEN.47                                                                             75  sPS-200                                                                            25  9.8   --    --   10  --   --                           56   PEN.47                                                                             75  sPS-320                                                                            25  9.8   --    --   12  --   --                           57   PEN.47                                                                             75  sPS-320                                                                            25  16.3  --    --   --  --   --                           58   PEN.47                                                                             95  sPS-320                                                                            5   9.8   --    --   --  --   --                           59   PEN.47                                                                             95  sPS-320                                                                            5   16.3  --    --   --  --   --                           60   PEN.56                                                                             100 --   --  16.3, 9.8                                                                           --    --   --  --   x                            61   PEN.56                                                                             75  sPS-200                                                                            25  9.8   --    --   10  --   --                           62   PEN.56                                                                             75  sPS-200                                                                            25  16.3  --    --   --  --   x                            63   PEN.56                                                                             95  sPS-200                                                                            5   9.8   --    --   --  --   --                           64   PEN.56                                                                             95  sPS-200                                                                            5   16.3  --    --   --  --   x                            65   PEN.56                                                                             75  sPS-320                                                                            25  9.8   --    --   10  --   --                           66   PEN.56                                                                             75  sPS-320                                                                            25  16.3  --    --   --  --   --                           67   PEN.47                                                                             95  sPS-200                                                                            5   16.3  2%    0.25%                                                                              1   0.3  x                                                         GMAPS2                                                                              ETPB                                       68   PEN.47                                                                             95  sPS-200                                                                            5   9.8   2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       69   PEN.56                                                                             75  sPS-200                                                                            25  9.8   6%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       70   PEN.56                                                                             75  sPS-200                                                                            25  16.3  6%    0.25%                                                                              0.5 2.5  x                                                         GMAPS2                                                                              ETPB                                       71   PEN.56                                                                             75  sPS-200                                                                            25  9.8   2%    0.25%                                                                              --  0.8  --                                                        GMAPS2                                                                              ETPB                                       72   PEN.56                                                                             75  sPS-200                                                                            25  16.3  2%    0.25%                                                                              1   --   --                                                        GMAPS2                                                                              ETPB                                       73   PEN.56                                                                             95  sPS-200                                                                            5   9.8   2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       74   PEN.56                                                                             95  sPS-200                                                                            5   16.3  2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       75   PEN.56                                                                             75  sPS-200                                                                            25  9.8   6%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       76   PEN.56                                                                             75  sPS-200                                                                            25  16.3  6%    0.25%                                                                              0.8 1    x                                                         GMAPS2                                                                              ETPB                                       77   PEN.56                                                                             75  sPS-200                                                                            25  9.8   2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       78   PEN.56                                                                             75  sPS-200                                                                            25  16.3  2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       79   PEN.56                                                                             75  sPS-200                                                                            25  9.8   6%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       80   PEN.56                                                                             75  sPS-200                                                                            25  16.3  6%    0.25%                                                                              --  --   x                                                         GMAPS2                                                                              ETPB                                       81   PEN.56                                                                             75  sPS-200                                                                            25  9.8   6%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       82   PEN.56                                                                             75  sPS-200                                                                            25  16.3  6%    0.25%                                                                              0.5 --   --                                                        GMAPS2                                                                              ETPB                                       83   PEN.56                                                                             95  sPS-200                                                                            5   9.8   2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       84   PEN.56                                                                             95  sPS-200                                                                            5   16.3  2%    0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       85   PEN.56                                                                             75  sPS-200                                                                            25  9.8   0.5%  0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       86   PEN.56                                                                             75  sPS-200                                                                            25  9.8   0.5%  0.25%                                                                              --  --   --                                                        GMAPS2                                                                              ETPB                                       87   PEN.47                                                                             75  Eastar                                                                             25  16.3  --    --   --  --   x                            88   PEN.47                                                                             75  Eastar                                                                             25  9.8   --    --   --  --   --                           89   PEN.47                                                                             75  Eastar                                                                             25  16.3  --    --   --  --   --                           90   PEN.47                                                                             75  Eastar                                                                             25  9.8   --    --   --  --   --                           91   PEN.47                                                                             75  PMMA 25  9.8   --    --   --  --   --                                         VO44                                                            92   PEN.47                                                                             75  PMMA 25  16.3  --    --   10  --   --                                         VO44                                                            93   PEN.47                                                                             75  PMMA 25  16.3  6%    --   --  0.7  --                                         VO44           MMA/GMA                                          94   PEN.47                                                                             75  PMMA 25  9.8   6%    --   --  --   --                                         VO44           MMA/GMA                                          95   PEN.47                                                                             75  PMMA 25  9.8   2%    --   --  1.2  --                                         VO44           MMA/GMA                                          96   PEN.47                                                                             75  PMMA 25  16.3  2%    --   --  --   x                                                         MMA/GMA                                          97   PEN.47                                                                             75  sPS-200-4                                                                          25  916.3 0.5% Congo                                                                          --   --  --   x                                          VO44           Red                                              98   PEN.47                                                                             75  sPS-200-4                                                                          25  16.3  0.15% --   --  --   x                                                         Congo Red                                        99   PEN.47                                                                             75  sPS-200-4                                                                          25  9.8   0.25% --   --  --   --                                                        Methylene                                                                     Blue                                             100  PEN.47                                                                             75  sPS-200-4                                                                          25  9.8   0-0.25%                                                                             --   --  --   --                                                        Methylene                                                                     Blue                                             __________________________________________________________________________

The presence of the various compatibilizers was found to reduce the sizeof the or disperse phase.

EXAMPLE 101

In Example 101, an optical film was made in a manner similar to Example4 except the resulting core thickness was about 420 microns thick, andeach skin layer was about 105 microns thick. The PEN had a 0.56 IV. Thecast film was oriented as in Example 1, except the temperature ofstretch was 165° C. and there was a 15 day delay between casting andstretching. The transmission was 87.1% and 39.7% for parallel andperpendicularly polarized light, respectively.

EXAMPLES 102-121

In Examples 102-121, optical films were made as in Example 101, exceptthat orientation conditions were varied and/or the sPS-200-0 wasreplaced with either copolymers of sPS containing either 4 or 8 mole %of para-methyl styrene or with an atactic-form of styrene, Styron 663(available from Dow Chemical Company, Midland, Mich.) as listed in Table3. Evaluations of transmission properties are ported. Transmissionvalues are averaged over all wavelengths between 450-700 nm.

                                      TABLE 3                                     __________________________________________________________________________                   Temperature                                                                         Rail Perpendicular                                                                        Parallel                                         %      PEN of Draw                                                                             Setting                                                                            Transmission                                                                         Transmission                                 Ex. sPS                                                                              PS  IV  (° C.)                                                                       (cm) (%)    (%)                                          __________________________________________________________________________    101 25 200-0                                                                             0.56                                                                              165   152  87.1   39.7                                         102 35 200-0                                                                             0.56                                                                              165   152  87.8   44.4                                         103 15 200-4                                                                             0.56                                                                              165   152  86.1   43.5                                         104 25 200-4                                                                             0.56                                                                              165   152  86.5   43.6                                         105 35 200-4                                                                             0.56                                                                              165   152  88.2   50.7                                         106 15 200-8                                                                             0.56                                                                              165   152  89.3   40.7                                         107 25 200-8                                                                             0.56                                                                              165   152  88.5   42.8                                         108 35 200-8                                                                             0.56                                                                              165   152  88.6   43.3                                         109 15 Styron                                                                            0.56                                                                              165   152  89.3   45.7                                                663                                                                    110 25 Styron                                                                            0.56                                                                              165   152  87.8   41.6                                                663                                                                    111 35 Styron                                                                            0.56                                                                              165   152  88.8   48.2                                                663                                                                    112 15 Styron                                                                            0.48                                                                              165   152  88.5   62.8                                                663                                                                    113 25 Styron                                                                            0.48                                                                              165   152  87.1   59.6                                                663                                                                    114 35 Styron                                                                            0.48                                                                              165   152  86.8   59.6                                                663                                                                    115 15 200-0                                                                             0.48                                                                              165   152  88.0   58.3                                         116 25 200-0                                                                             0.48                                                                              165   152  88.0   58.7                                         117 35 200-0                                                                             0.48                                                                              165   152  88.5   60.6                                         118 15 200-4                                                                             0.48                                                                              165   152  89.0   57.4                                         119 35 200-4                                                                             0.48                                                                              165   152  87.3   64.0                                         120 35 200-0                                                                             0.56                                                                              171   127  86.5   65.1                                         121 35 200-0                                                                             0.56                                                                              171   152  88.1   61.5                                         __________________________________________________________________________

These examples indicate that the particles of the included phase areelongated more in the machine direction in high IV PEN than in low IVPEN. This is consistent with the observation that, in low IV PEN,stretching occurs to a greater extent near the surface of the film thanat points interior to the film, with the result that fibrillarstructures are formed near the surface and spherical structures areformed towards the center.

Some of these Examples suggest that the orientation temperatures anddegree of orientation are important variables in achieving the desiredeffect. Examples 109 to 114 suggest that quiescent crystallization neednot be the only reason for the lack of transmission of a preferredpolarization of light.

EXAMPLES 122-124

In Example 122, a multilayer optical film was made in accordance withthe invention by means of a 209 layer feedblock. The feedblock was fedwith two materials: (1) PEN at 38.6 kg per hour (intrinsic viscosity of0.48); and (2) a blend of 95% CoPEN and 5% by weight of sPS homopolymer(200,000 molecular weight). The CoPEN was a copolymer based on 70 mole %naphthalene dicarboxylate and 30 mole % dimethyl isophthalatepolymerized with ethylene glycol to an intrinsic viscosity of 0.59. TheCoPEN/sPS blend was fed into the feedblock at a rate of 34.1 kg perhour.

The CoPEN blend material was on the outside of the extrudate, and thelayer composition of the resulting stack of layers alternated betweenthe two materials. The thicknesses of the layers was designed to resultin a one-quarter wavelength stack with a linear gradient of thicknesses,and having a 1.3 ratio from the thinnest to the thickest layer. Then, athicker skin layer of CoPEN (made in accordance with the methoddescribed above to make the CoPEN/sPS blend, except the molar ratioswere 70/15/15 naphthalene dicarboxylate/dimethyl terephthalate/dimethylisophthalate) devoid of sPS was added to each side of the 209 layercomposite. The total skin layer was added at a rate of 29.5 kg per hour,with about one-half of this quantity on each side or surface of thestack.

The resulting skin layer clad multilayer composite was extruded througha multiplier to achieve a multilayer composite of 421 layers. Theresulting multilayer composite was then clad with another skin layer ofthe 70/15/15 CoPEN on each surface at a total rate of 29.5 kg per hourwith about one-half of this quantity on each side. Since this secondskin layer may not be separately detectable from the existing skin layer(as the material is the same), for the purposes of this discussion, theresulting extra thick skin layer will be counted as only one layer.

The resulting 421 layer composite was again extruded through a 1.40ratio asymmetric multiplier to achieve a 841 layer film which was thencast into a sheet by extruding through a die and quenching into a sheetabout 30 mils thick. The resulting cast sheet was then oriented in thewidth direction using a conventional film making tentering device. Thesheet was stretched at a temperature of about 300° F. (149° C.) to astretch ratio of about 6:1 and at a stretch rate of about 20% persecond. The resulting stretched film was about 5 mils thick.

In Example 123, a multilayer optical film was made as in Example 122,except that the amount of sPS in the CoPEN/sPS blend was 20% instead of5%.

In Example 124, a multilayer optical film was made as in Example 122,except that no sPS was added to the film.

The results reported in Table 4 include a measure of the optical gain ofthe film. The optical gain of a film is the ratio of light transmittedthrough an LCD panel from a backlight with the film inserted between thetwo to the light transmitted without the film in place. The significanceof optical gain in the context of optical films is described in WO95/17692 in relation to FIG. 2 of that reference. A higher gain value isgenerally desirable. The transmission values include values obtainedwhen the light source was polarized parallel to the stretch direction(T.sub.∥) and light polarized perpendicular to the stretch direction(T.sub.⊥). Off-angle-color (OAC) was measured using an Orielspectrophotometer as the root mean square deviation of p-polarizedtransmission at 50 degree incident light of wavelength between 400 and700 nm.

                  TABLE 4                                                         ______________________________________                                               mole %                                                                 Ex.    sPS      Gain   T.sub.⊥  (%)                                                                     T.sub.|  (%)                                                               OAC (%)                                  ______________________________________                                        122    5        1.5    83      2     1.5                                      123    20       1.45   81      1.5   1.2                                      124    0        1.6    87      5     3.5                                      ______________________________________                                    

The value of off-angle-color (OAC) demonstrates the advantage of using amultilayer construction within the context of the present invention. Inparticular, such a construction can be used to substantially reduce OACwith only a modest reduction in gain. This tradeoff may have advantagesin some applications. The values of T.sub.∥ for the examples of theinvention may be lower than expected because light scattered by the sPSdispersed phase may not be received by the detector.

The preceding description of the present invention is merelyillustrative, and is not intended to be limiting. Therefore, the scopeof the present invention should be construed solely by reference to theappended claims.

What is claimed is:
 1. A method for making a polarizer, comprising thesteps of:creating a blend of first and second diverse thermoplasticpolymers, wherein said first thermoplastic polymer is present as acontinuous phase and is capable of undergoing strain-inducedbirefringence, and wherein said second thermoplastic polymer is presentas a disperse phase; and orienting the blend in at least one directionsuch that the first thermoplastic polymer develops a birefringence of atleast about 0.05;wherein the blend is stretched in a first direction toa first stretch ratio k and is stretched in a second direction to asecond stretch ratio m, and wherein k>m.
 2. The method of claim 1,wherein the blend is oriented such that it has a total diffusereflectivity of at least 30% over essentially all wavelengths of visiblelight.
 3. The method of claim 1, wherein m is at least about
 2. 4. Themethod of claim 1, wherein m is at least about
 4. 5. The method of claim1, wherein m is at least about
 6. 6. The method of claim 1, wherein theblend is extruded into a film before it is oriented, wherein the firstdirection is along a first in-plane axis, and wherein the seconddirection is along a second in-plane axis.
 7. The method of claim 6,wherein the first and second in-plane axes are essentially orthogonal.8. The method of claim 1, wherein the disperse phase is present in anamount of about 5% to about 50% by volume relative to the volume of thecontinuous phase.
 9. A method for making a polarizer, comprising thesteps of:creating a blend comprising first and second diversethermoplastic polymers, wherein the first thermoplastic polymer ispresent as a continuous phase and is capable of undergoingstrain-induced birefringence, and wherein the second thermoplasticpolymer is present in the blend as a disperse phase which is essentiallyrandomly distributed throughout the continuous phase; extruding theblend into a film; and orienting the film in at least one direction suchthat the first thermoplastic polymer develops a birefringence of atleast about 0.05;wherein the film is oriented by stretching it in afirst in-plane direction while leaving the film unconstrained in asecond in-plane direction.
 10. The method of claim 9, wherein the secondin-plane direction is essentially orthogonal to the first in-planedirection.
 11. The method of claim 9, wherein the blend is oriented suchthat it has a total diffuse reflectivity of at least 30% overessentially all wavelengths of visible light.
 12. The method of claim 9,wherein the blend is stretched in the first in-plane direction to astretch ratio of at least about
 2. 13. The method of claim 9, whereinthe blend is stretched in the first in-plane direction to a stretchratio of at least about
 4. 14. The method of claim 9, wherein the blendis stretched in the first in-plane direction to a stretch ratio of atleast about
 6. 15. The method of claim 9, wherein the second in-planedirection is orthogonal to the first in-plane direction.
 16. The methodof claim 9, wherein said disperse phase is present in an amount of about5% to about 50% by volume relative to the volume of the continuousphase.
 17. A method for making a polarizer, comprising the stepsof:creating a blend comprising first and second diverse thermoplasticpolymers and a compatibilizer, wherein the first thermoplastic polymeris present as a continuous phase and is capable of undergoingstrain-induced birefringence, and wherein the second thermoplasticpolymer is present in the blend as a disperse phase which is essentiallyrandomly distributed throughout the continuous phase; extruding theblend into a film; and stretching the film in at least one directionsuch that the first thermoplastic polymer develops a birefringence of atleast about 0.05.
 18. The method of claim 17, wherein the stretched filmhas a total diffuse reflectivity of at least 30% over essentially allwavelengths of visible light.
 19. The method of claim 17, wherein thefilm is stretched to a stretch ratio of at least about
 2. 20. The methodof claim 17, wherein the film is stretched to a stretch ratio of atleast about
 4. 21. The method of claim 17, wherein the film is stretchedto a stretch ratio of at least about
 6. 22. The method of claim 17,wherein the film is stretched in a first direction to a stretch ratio kand is stretched in a second direction to a stretch ratio m, and whereink>m.
 23. The method of claim 17, wherein the film is stretched in afirst in-plane direction to a stretch ratio of at least 2, while leavingthe film unconstrained in a second in-plane direction.
 24. The method ofclaim 23, wherein the second in-plane direction is orthogonal to thefirst in-plane direction.
 25. The method of claim 17, wherein saiddisperse phase is present in an amount of about 5% to about 50% byvolume relative to the volume of the continuous phase.
 26. A method formaking a polarizer, comprising the steps of:creating a blend comprisingfirst, second, and third diverse polymers, wherein the first and secondpolymers are thermoplastic, wherein the first polymer is present in theblend as a continuous phase and is capable of undergoing strain-inducedbirefringence, and wherein the second and third polymers are present inthe blend as disperse phases which are essentially randomly distributedthroughout the continuous phase; and stretching the blend in at leastone direction such that the first thermoplastic polymer develops abirefringence of at least about 0.05.
 27. The method of claim 26,wherein the blend is oriented such that it has a total diffusereflectivity of at least 30% over essentially all wavelengths of visiblelight.
 28. The method of claim 26, wherein the blend is stretched to astretch ratio of at least about
 2. 29. The method of claim 26, whereinthe blend is stretched to a stretch ratio of at least about
 4. 30. Themethod of claim 26, wherein the blend is stretched to a stretch ratio ofat least about
 6. 31. The method of claim 26, wherein the blend isstretched in a first direction to a stretch ratio k, wherein the blendis stretched in a second direction to a stretch ratio m, and whereink>m.
 32. The method of claim 26, wherein the blend is extruded as a filmprior to being stretched, and wherein the film is stretched in a firstin-plane direction to a stretch ratio of at least 2 while leaving thefilm unconstrained in a second in-plane direction.
 33. The method ofclaim 26, wherein said disperse phase is present in an amount of about5% to about 50% by volume relative to the volume of the continuousphase.
 34. The method of claim 26, wherein the second in-plane directionis orthogonal to the first in-plane direction.
 35. A method for making apolarizer, comprising the steps of:creating a blend comprising first andsecond diverse thermoplastic polymers, wherein the first thermoplasticpolymer is present as a continuous phase and is capable of undergoingstrain-induced birefringence, and wherein the second thermoplasticpolymer is present in the blend as a disperse phase which is essentiallyrandomly distributed throughout the continuous phase; extruding theblend into a film; and stretching the film asymmetrically and biaxiallysuch that the first thermoplastic polymer develops a birefringence of atleast about 0.05.
 36. The method of claim 35, wherein the film isstretched such that it has a total diffuse reflectivity of at least 30%over essentially all wavelengths of visible light.
 37. The method ofclaim 35, wherein the film is stretched to a stretch ratio of at leastabout
 2. 38. The method of claim 35, wherein the film is stretched to astretch ratio of at least about
 4. 39. The method of claim 35, whereinthe film is stretched to a stretch ratio of at least about
 6. 40. Themethod of claim 35, wherein the film is stretched in a first directionto a stretch ratio k, wherein the film is stretched in a seconddirection to a stretch ratio m, and wherein k>m.
 41. The method of claim40, wherein the second in-plane direction is orthogonal to the firstin-plane direction.
 42. A method for making a polarizer, comprising thesteps of:creating a blend comprising first and second diversethermoplastic polymers, wherein the first thermoplastic polymer ispresent as a continuous phase and is capable of undergoingstrain-induced birefringence, and wherein the second thermoplasticpolymer is present in the blend as a disperse phase which is essentiallyrandomly distributed throughout the continuous phase; extruding theblend into a film; and stretching the film in at least one directionsuch that the first thermoplastic polymer develops a birefringence of atleast about 0.05;wherein the blend is extruded as a first layer in arecurrent sequence of at least two layers.
 43. The method of claim 42,wherein the film is stretched such that it has a total diffusereflectivity of at least 30% over essentially all wavelengths of visiblelight.
 44. The method of claim 42, wherein the film is stretched to astretch ratio of at least about
 2. 45. The method of claim 42, whereinthe film is stretched to a stretch ratio of at least about
 4. 46. Themethod of claim 42, wherein the film is stretched to a stretch ratio ofat least about
 6. 47. The method of claim 42, wherein the film isstretched in a first direction to a stretch ratio k, wherein the film isstretched in a second direction to a stretch ratio m, and wherein k>m.48. The method of claim 42, wherein the film is stretched in a firstin-plane direction to a stretch ratio of at least 2 while leaving thefilm unconstrained in a second in-plane direction.
 49. The method ofclaim 48, wherein the second in-plane direction is orthogonal to thefirst in-plane direction.